What methods can I use to reduce such a visible seam?
There are 2 methods worth looking into.


Blended box mapping (cycles / rendering only)Easy to enable and apply everywhere but adds noticeable blurring/blending.
Projection painting - use the clone tool in 3d to clone over the seamGives a lot of control but makes the texture specific to meshes UV's, and wont work for tiling textures. However it can be used anywhere UV textures are supported - exporting to game engines for eg.